Spread Spectrum (SS) is a communication scheme that is advantageous for several applications. In the past, spread spectrum was used in military applications because of its resistance against jamming. More recently, spread spectrum has formed the basis of Code-Division Multiple Access (CDMA) communication systems, some which have been applied in cellular radio telephone environments due to its advantageous resistance to fading.
In a typical CDMA system, an information datastream to be transmitted is impressed upon a much higher bitrate datastream generated by, e.g., a pseudorandom code generator. The information datastream and the higher bitrate datastream are typically multiplied together, and such combination of the higher bitrate signal with the lower bitrate information signal is called direct-sequence (DS) spreading of the signal. Each information datastream or channel is allocated a unique spreading code. A plurality of SS signals are transmitted upon radio frequency carrier waves and jointly received as a composite signal at a receiver. Each of the SS signals overlaps all of the other SS signals, as well as noise related signals, in both frequency and time. By correlatirg the composite signal with one of the unique spreading codes, the corresponding information signal can be isolated and despread at the receiver.
The receiver processes the received signal to produce an estimate of the original message signal. This process is referred to as demodulation. In a Direct Sequence CDMA system, demodulation is commonly performed in a RAKE receiver. A RAKE receiver is a type of receiver where several propagation paths can be detected and combined, or "raked", together before decoding. These different propagation paths of a radio signal occur because of reflections from buildings or other large nearby obstacles. This combination or "raking" is an advantageous way of utilizing as much of the transmitted energy as possible in the detection.
A detector can either operate to coherently detect a received signal or it may operate to non-coherently detect a received signal. In a coherent detection scheme the channel response is determined such that the effects of phase and magnitude distortions caused by the communication channel can be compensated for with matched filters. This is typically done by first transmitting a pilot signal. For example, in a cellular communication system, the forward channel, or down-link, may be coherently detected if the base station transmits a pilot signal. This is a known signal and the receiver at the mobile stations can then use this pilot signal to estimate the channel phase and magnitude parameters, to subsequently perform a coherent detection.
In a non-coherent detection scheme however, there is no compensation for phase distortions. For diversity reception, there is thus a non-negligible combining loss in non-coherent detection schemes, Consequently, a coherent detection requires typically less signal to noise ratio than that required by a non-coherent detector for the same bit error rate.
In an uplink channel, from mobile to base station, using a pilot signal may not be feasible. For example, the CDMA system specified by the TIA/EIA/IS-95 standard promulgated by the Telecommunications Industry Association and the Electronic Industries Association uses Direct Sequence spreading and non-coherent detection in the uplink. The IS-95 standard specifies conventional CDMA, in which each user demodulates its received signal without considering other users' signals, in a cellular communication system.
A typical spread spectrum transmission involves expanding the bandwidth of an information signal, transmitting the expanded signal, and recovering the desired information signal by remapping the received spread spectrum signal into the original information signal's bandwidth. The quality of the recovery of the transmitted information signal from the commuication channel is measured by the error rate for the energy per bit over noise spectral density, E.sub.b /N.sub.0. As the error rate increases, the quality of the signal received by the receiving party decreases. Most communication systems are designed to limit the error rate to an upper bound, or maximum, so that degradation of the received signal is limited.
In a Direct Sequence CDMA system, e.g., IS-95, the error rate is related to several factors. One of these is the interference level of the channel, which is directly related to the number of simultaneous users within the same frequency bandwidth. A received signal intended for a particular mobile station is experienced as interference to all other mobile stations receiving within the same bandwidth in a cell.
The error rate is also affected by the received signal power level. In some spread spectrum systems (e.g. cellular systems ) a central communications site attempts to detect or receive more than one signal from a particular bandwidth of the spectrum. This could typically be a base station. The site then adjusts its receiver components to optimally receive signals at a particular received signal power threshold level. The signals having a received power at or near the threshold level are optimally received while those signals not having a received power at or near the threshold level are not optimally received.
If all the mobile transmitters' powers received at a receiver are equal to one another, the signal to noise ratio can be maintained above the threshold by not allowing the number of mobile stations in a cell to exceed a certain number. Then, the reception is optimal from a system view, not necessarily from an individual user's viewpoint. Optimal can be defined, for example, to be the maximum number of users at a particular maximum error rate. In this sense, a non-optimally received signal tends to have a higher error rate or cause unnecessary interference to other receivers. Either of these consequences can result in the system further limiting the number of simultaneous users in the frequency bandwidth associated with a particular site.
Thus it is desirable to maintain the received signal power level at or near the particular power threshold level. This can be done by adjusting the transmitted signal power level. By using power control schemes to maintain the received signal power levels at a particular power level, the number of simultaneous users can be maximized for a particular maximum error rate.
As mentioned above, particular transmitted signals in a CDMA cellular system can be retrieved by despreading. A composite signal representative of the sum of signals in a certain frequency bandwidth can be despread with user specific spreading codes related to a particular transmitted signal which is to be retrieved. When user specific spreading codes are orthogonal to one another, the received signal can be correlated with a particular user spreading code such that only the desired user signal related to a particular spreading code is enhanced while the other signals for all the other users will not be enhanced.
Various spreading codes exist which can be used to separate data signals from one another in a CDMA system. Also, some types of codes can be utilized for coding the information signal prior to modulation. Data signals are often channel coded to enable transmitted signals to better withstand the effects of various channel impairments, such as noise, fading, and jamming. One method is to have one code symbol correspond to one modulation symbol. This is called coded modulation.
One type of orthogonal code, which can be used for both of the above, is a Walsh code. A Walsh code corresponds to a single row or column in a Hadamard matrix. Walsh codes are orthogonal and have zero cross correlation. They are used both for user separation and coded modulation. For example in the uplink of an IS-95 system, M-ary orthogonal modulation with M=64 utilizing Walsh symbols is specified. On the other hand, in IS-95 downlink, Walsh codes are used for channel separation.
Walsh codes, or sequences, are powerful to use because there exists easily implemented methods of performing correlation calculations. These are usually performed in a Fast Walsh Transform (FWT), or Fast Hadamard Transform (FHT), a function which correlates each input Walsh symbol against all possible Walsh symbols. The output of the FWT/FHT is M correlation values, where M corresponds to the number of possible Walsh symbols. E.g., in 64-ary orthogonal modulation, the number of possible Walsh symbols is 64. One type of FWT is described in U.S. Pat. No. 5,357,454 to Dent for "Fast Walsh Transform Processor".
As mentioned above, the number of simultaneous users within the same frequency bandwidth is limited. The performance of the system is highly dependent on the received power of a certain signal This means that accurate power control is especially important for DS-CDMA communication systems, and thus there is a need to accurately estimate the received power in a receiver for use as input in a power control algorithm.
In FIG. 1 is generally shown part of a RAKE receiver and a power estimation function as could be implemented in a base station in a DS-CDMA communication system employing non-coherent detection. The receiver receives different signal paths and passes the signals through different delay lines D.sub.1, . . . D.sub.p, in 102 to align them in time. Further, a multiplication with the user specific PN sequences is performed 104 to retrieve a certain user3 s signal. Next, an integration is performed 106 over the time for a Walsh symbol T.sub.s followed by a Fast Walsh Transform and generation of complex correlation values indicating the correlation between the received Walsh words and all possible Walsh words.
It should here be noted that if some non-coherent modulation method is applied other than M-ary orthogonal modulation , of course no Walsh Transform is performed. One such example is a system employing Differential Binary Phase Shift Keying, DBPSK.
FIG. 1 is representative of such a scheme as well, and although the following description will be held mainly with respect to M-ary orthogonal modulation, other non-coherent modulation schemes are also considered.
For each received signal path, i.e., each "finger" in the RAKE receiver, the complex correlation values are absolute squared 108 and combined 110 with the corresponding symbol values for the other received signal paths, and a decision variable is generated at defection block 112 that is calculated based on what symbol was most likely sent.
As indicated in FIG. 1, the input to the power estimator 114 is based on values of the received signal after squaring and combining. Usually, power estimation is performed by averaging m consecutive detected symbols. This averaging is thus performed non-coherently, since no phase information is present in a squared channel complex estimate. The power estimation 114 for M-ary modulation is usually calculated as; ##EQU1## where p is the number of detected paths and .sup.Y.sbsp.i,j is the jth vector at delay D.sub.i consisting of a real and imaginary value for every possible symbol that can be transmitted plus noise. This power estimation method tends to generate overestimates of the received signal power and the variance also tends to be large.
For the case of a system employing a DBPSK scheme, the power estimation will be calculated as ##EQU2## where .sup..chi.,j is the jth complex amplitude of the channel, at delay D.sub.i modulated with the transmitted bit plus noise and p is the number of RAKE fingers currently used.
In addition, other power estimation methods for CDMA communication systems exist in the prior art. For example, in U.S. Pat. No. 5,297,161 to Ling for "Method and Apparatus for Power Estimation in an Orthogonal Coded Commuication System" is described one way of performing power estimation of received orthogonal symbols in non-coherent detection schemes by correlating an input data vector of the received signal with a set of mutually orthogonal codes to generate a set of output values. Each correlation value corresponds to a measure of confidence that the input data vector is substantially similar to one of the orthogonal codes from within the set of mutually orthogonal codes.
An estimate of the power of the received signal is generated as a nonlinear function of the set of output values. The largest of these values is then chosen. These largest values gives estimates of the signal power and they are then non-coherently averaged over six received Walsh symbols. This is the same procedure as the above, illustrated in FIG. 1. In addition, the other 63 values for every Walsh symbol are used to provide estimates of the noise power. These values are then used to compensate the average estimated signal power according to a particular function.
The method relies on estimation of power for the wanted signal, compensating for the power of the non-wanted signals. This means that a variance is calculated wherein other users' contributions to the overall noise have been considered. The estimation is performed over 1.25 ms non-coherent averaging periods, i.e. a time corresponding to 6 Walsh words. Without the compensation function the overestimation of the power is significant. By the compensation function the accuracy of the estimates is increased. The power estimation method in the Ling patent is illustrated in FIG. 2. The RAKE receiver scheme is the same as that illustrated in FIG. 1.